postfix config

the postfix documentation states the following:
"NEVER list a virtual alias domain name as a mydestination domain!"
Shouldn't it use virtual_alias_domains and virtual_alias_maps and just leave local-host-names alone?

Now why does ISPConfig do it that way? Is it for legacy purposes, i.e. to continue supporting Postfix 1.x?

Thanks for the tip.
However I do not see how mode 2 would help me either.
I tried it and the only thing it seems to do is write VIRTUALDOMAIN into virtualusertable, which makes no real sense to me... And it removed the domains from local-host-names, which is ok, but I would have expected a virtualdomainmap with the domains and a virtualusermap with the users...
So I don't see where mode 2 is going, maybe you can explain.
Anyway... I will switch back to mode 1 now

If you compile Postfix manually, you will find the file /etc/postfix/virtual which has the following information:

#
# VIRTUAL(5) VIRTUAL(5)
#
# NAME
# virtual - format of Postfix virtual table
#
# SYNOPSIS
# postmap /etc/postfix/virtual
#
# DESCRIPTION
# The optional virtual table specifies address redirections
# for local and non-local recipients or domains. The redi-
# rections are used by the cleanup(8) daemon. The redirec-
# tions are recursive.
#
# The virtual redirection is applied only to recipient enve-
# lope addresses, and does not affect message headers.
# Think Sendmail rule set S0, if you like. Use canonical(5)
# mapping to rewrite header and envelope addresses in gen-
# eral.
#
# Normally, the virtual table is specified as a text file
# that serves as input to the postmap(1) command. The
# result, an indexed file in dbm or db format, is used for
# fast searching by the mail system. Execute the command
# postmap /etc/postfix/virtual in order to rebuild the
# indexed file after changing the text file.
#
# When the table is provided via other means such as NIS,
# LDAP or SQL, the same lookups are done as for ordinary
# indexed files.
#
# Alternatively, the table can be provided as a regular-
# expression map where patterns are given as regular expres-
# sions. In that case, the lookups are done in a slightly
# different way as described below.
#
# POSTFIX-STYLE VIRTUAL DOMAINS
# With a Postfix-style virtual domain, the virtual domain
# has its own user name space. Local (i.e. non-virtual)
# usernames are not visible in a Postfix-style virtual
# domain. In particular, local aliases(5) and mailing lists
# are not visible as localname@virtual.domain.
#
# Use a Sendmail-style virtual domain (see below) if local
# usernames, aliases(5) or mailing lists should be visible
# as localname@virtual.domain.
#
# Support for a Postfix-style virtual domain looks like:
#
# /etc/postfix/virtual:
# virtual.domain anything (right-hand content does not matter)
# postmaster@virtual.domain postmaster
# user1@virtual.domain address1
# user2@virtual.domain address2, address3
#
# The virtual.domain anything entry is required for a Post-
# fix-style virtual domain.
#
# Do not list a Postfix-style virtual domain in the main.cf
# mydestination configuration parameter. Such an entry is
# required only for a Sendmail-style virtual domain.
#
# With a Postfix-style virtual domain, the Postfix SMTP
# server accepts mail for known-user@virtual.domain and
# rejects mail for unknown-user@virtual.domain as undeliver-
# able.
#
# SENDMAIL-STYLE VIRTUAL DOMAINS
# With a Sendmail-style virtual domain, every local (i.e.
# non-virtual) username is visible in the virtual domain. In
# particular, every local alias and mailing list is visible
# as localname@virtual.domain.
#
# Use a Postfix-style virtual domain (see above) if local
# usernames, aliases(5) or mailing lists should not be visi-
# ble as localname@virtual.domain.
#
# Support for a Sendmail-style virtual domain looks like:
#
# /etc/postfix/main.cf:
# mydestination = $myhostname localhost.$mydomain $mydomain
# virtual.domain
#
# /etc/postfix/virtual:
# user1@virtual.domain address1
# user2@virtual.domain address2, address3
#
# The main.cf mydestination entry is required for a Send-
# mail-style virtual domain.
#
# Do not specify a virtual.domain anything virtual map entry
# for a Sendmail-style virtual domain. Such an entry is
# required only with a Postfix-style virtual domain.
#
# With a Sendmail-style virtual domain, the Postfix local
# delivery agent delivers mail for an unknown user@vir-
# tual.domain to a local (i.e. non-virtual) user that has
# the same name; if no such recipient exists, the Postfix
# local delivery agent bounces the mail to the sender.
#
# TABLE FORMAT
# The format of the virtual table is as follows, mappings
# being tried in the order as listed in this manual page:
#
# pattern result
# When pattern matches a mail address, replace it by
# the corresponding result.
#
# blank lines and comments
# Empty lines and whitespace-only lines are ignored,
# as are lines whose first non-whitespace character
# is a `#'.
#
# multi-line text
# A logical line starts with non-whitespace text. A
# line that starts with whitespace continues a logi-
# cal line.
#
# With lookups from indexed files such as DB or DBM, or from
# networked tables such as NIS, LDAP or SQL, patterns are
# tried in the order as listed below:
#
# user@domain address, address, ...
# Mail for user@domain is redirected to address.
# This form has the highest precedence.
#
# user address, address, ...
# Mail for user@site is redirected to address when
# site is equal to $myorigin, when site is listed in
# $mydestination, or when it is listed in
# $inet_interfaces.
#
# This functionality overlaps with functionality of
# the local alias(5) database. The difference is that
# virtual mapping can be applied to non-local
# addresses.
#
# @domain address, address, ...
# Mail for any user in domain is redirected to
# address. This form has the lowest precedence.
#
# In all the above forms, when address has the form @other-
# domain, the result is the same user in otherdomain. This
# works for the first address in the expansion only.
#
# ADDRESS EXTENSION
# When a mail address localpart contains the optional recip-
# ient delimiter (e.g., user+foo@domain), the lookup order
# becomes: user+foo@domain, user@domain, user+foo, user, and
# @domain. An unmatched address extension (+foo) is propa-
# gated to the result of table lookup.
#
# REGULAR EXPRESSION TABLES
# This section describes how the table lookups change when
# the table is given in the form of regular expressions. For
# a description of regular expression lookup table syntax,
# see regexp_table(5) or pcre_table(5).
#
# Each pattern is a regular expression that is applied to
# the entire address being looked up. Thus, user@domain mail
# addresses are not broken up into their user and @domain
# constituent parts, nor is user+foo broken up into user and
# foo.
#
# Patterns are applied in the order as specified in the
# table, until a pattern is found that matches the search
# string.
#
# Results are the same as with indexed file lookups, with
# the additional feature that parenthesized substrings from
# the pattern can be interpolated as $1, $2 and so on.
#
# BUGS
# The table format does not understand quoting conventions.
#
# CONFIGURATION PARAMETERS
# The following main.cf parameters are especially relevant
# to this topic. See the Postfix main.cf file for syntax
# details and for default values. Use the postfix reload
# command after a configuration change.
#
# virtual_maps
# List of virtual mapping tables.
#
# Other parameters of interest:
#
# inet_interfaces
# The network interface addresses that this system
# receives mail on.
#
# mydestination
# List of domains that this mail system considers
# local.
#
# myorigin
# The domain that is appended to locally-posted mail.
#
# owner_request_special
# Give special treatment to owner-xxx and xxx-request
# addresses.
#
# SEE ALSO
# cleanup(8) canonicalize and enqueue mail
# postmap(1) create mapping table
# pcre_table(5) format of PCRE tables
# regexp_table(5) format of POSIX regular expression tables
#
# LICENSE
# The Secure Mailer license must be distributed with this
# software.
#
# AUTHOR(S)
# Wietse Venema
# IBM T.J. Watson Research
# P.O. Box 704
# Yorktown Heights, NY 10598, USA
#
#
#

Well, I tried it and it didn't seem to work.
What they tell us in http://www.postfix.org/VIRTUAL_README.html
at "Postfix virtual ALIAS example: separate domains, UNIX system accounts"
seems to work out of the box...
Regards,
~jm

Yeah it's ok as far I'm concerned but it is not very elegant for newer version of Postfix. Maybe at some time they will discontinue the support for this, which is needed to stay compatible to 1.x.
Anyway, the sendmail style in ISPConfig is what I am using, and it works alright. I didn't get the postfix style to work but maybe I did something else wrong. Never mind...